1. Field of the Invention
The invention relates to an abnormality determination apparatus that is applied to an intake amount control mechanism of an internal combustion engine which includes a throttle valve and a changing mechanism capable of changing the lift working angle of an intake valve so as to make determination on the presence or absence of abnormality regarding the intake amount control mechanism.
2. Description of the Related Art
The adjustment of the amount of intake of an internal combustion engine is carried out through the adjustment of the opening amount of a throttle valve provided in an intake passageway. The throttle valve is driven and controlled so that the actual opening amount of the throttle valve becomes equal to a control target opening amount that is set on the basis of an index value regarding an engine operation state, such as the operation amount of accelerator, or the like.
Examples of known apparatuses for determining abnormality of an intake amount control mechanism as described above include an apparatus that determines that there is an abnormality in the throttle valve or its drive mechanism, on the condition that a state where the control target opening amount of the throttle valve and the actual opening amount thereof are apart from each other continues for a predetermined duration. Such related-art technologies are disclosed in, for example, Japanese Patent Application Laid-Open Publication No. JP-A-5-99002, or Japanese Patent Application Laid-Open Publication No. JP-A-7-12001.
A recently proposed mechanism for adjusting the amount of intake in an internal combustion engine is a technology in which, in addition to the mechanism for changing the state of the throttle valve, a mechanism for changing the duration from the opening of the intake valve till the closing thereof (lift working angle) in accordance with the engine operation state is provided. In such intake amount control mechanisms, it is difficult to simply set, for all conditions, a control target opening amount of the throttle valve on the basis of an index value of the engine operation state, for the following reasons.
A downstream portion of the intake passageway that extends from the throttle valve to an engine combustion chamber is provided with members that have large capacities, such as a surge tank, and the like. Therefore, in an internal combustion engine provided with the above-described intake amount control mechanism, the pressure in such a downstream portion changes in accordance with the lift working angle of the intake valve. The amount of intake air that passes through the throttle valve greatly varies depending on the pressure difference between the upstream side and the downstream side of the valve. Therefore, in order to set the aforementioned control target opening amount at a value that corresponds to the engine operation state, it is necessary to factor in the lift working angle of the intake valve as well, which causes changes in the pressure difference as mentioned above.
Taking this point into consideration, in an internal combustion engine provided with the above-described intake amount control mechanism, the control target opening amount of the throttle valve is ordinarily set at a value that corresponds to the then-occurring lift working angle. Therefore, in the case of related-art apparatuses that determine abnormality regarding the driving of the throttle valve provided that the state where the control target opening amount set on the basis of the amount of accelerator operation or the like, and the actual opening amount are apart from each other continues for a predetermined time, false determinations are inevitable depending on the situation regarding the lift working angle of the intake valve.
Furthermore, since the appropriate opening amount of the throttle valve or the appropriate lift working angle of the intake valve changes depending on the states of the opening amount and the lift working angle as described above, the determination on abnormality regarding the driving of the intake valve has a similar difficulty, that is, false determinations are inevitable depending on the opening amount of the throttle valve.